1. Field of the Invention
The present invention relates to the formation of integrated circuits, and, more particularly, to the inspection of semiconductor structures.
2. Description of the Related Art
Integrated circuits comprise a large number of individual circuit elements, such as transistors, capacitors and resistors, formed on a substrate. These elements are connected internally by means of electrically conductive lines to form complex circuits, such as memory devices, logic devices and microprocessors. The performance of integrated circuits can be improved by increasing the number of functional elements per circuit, in order to increase their functionality, and/or by increasing the speed of operation of the individual circuit elements. A reduction of feature sizes allows the formation of a greater number of circuit elements on the same area, hence allowing an extension of the functionality of the circuit, and also reduces signal propagation delays. The reduction of signal propagation times allows an increase of the speed of operation of the circuit elements. In modern integrated circuits, design rules of about 90 nm or less can be applied.
The formation of integrated circuits comprises a deposition of a large number of material layers on a semiconductor structure. The material layers are patterned by means of photolithography and etching in order to form the circuit elements and the components thereof.
Since characteristics of the material layers may strongly influence the performance of the integrated circuit, the formation of integrated circuits having small feature sizes requires a precise and reproducible deposition of material layers on a semiconductor substrate. Therefore, considerable effort is applied to the development of processes for the deposition of material layers. The development of such processes requires tools and methods for the characterization of the deposited material layers.
Frequently, electron microscopy is applied for this purpose. In addition to obtaining high resolution images of the semiconductor structure, electron microscopy may also be employed in determining a chemical composition of the semiconductor structure and material layers formed thereon. In the following, a method of inspecting a semiconductor structure according to the state of the art to determine the chemical composition of a material layer on the semiconductor structure will be described with reference to FIGS. 1a and 1b. 
FIG. 1a shows a schematic perspective view of a section of a semiconductor structure 100. The semiconductor structure 100 comprises a substrate 101. On the substrate 101, a first material layer 102, a second material layer 103 and a third material layer 104 are formed. As persons skilled in the art know, the semiconductor structure 100 can be formed by performing a variety of known deposition methods, including chemical vapor deposition, plasma enhanced chemical vapor deposition and/or physical vapor deposition.
A semiconductor sample 120 (FIG. 1b) provided in the form of a cross-sectional specimen of the semiconductor structure 100 is prepared. To this end, the semiconductor structure 100 is cut along lines 105, 106 which can be substantially parallel to each other. A direction of the cuts indicated by an arrow 121 is substantially perpendicular to the surface of the substrate 101 and the material layers 102, 103, 104 formed thereon. Cutting the semiconductor structure 100 can be performed by means of mechanical milling techniques which are known to persons skilled in the art. After the cutting, a thickness of the semiconductor sample 120 may be reduced by means of advanced methods of ion beam milling and polishing techniques also known to persons skilled in the art.
FIG. 1b shows a schematic perspective view of an electron microscope 130. The electron microscope 130 comprises an electron source 107, an electron optic 109, a detector 110 and a sample holder 121. These components are provided inside a vacuum chamber (not shown). The semiconductor sample 120 is attached to the sample holder 121.
The electron optic 109 is configured to focus an electron beam 108 provided by the electron source 107 to the semiconductor sample 120. The detector 110 is configured to measure a property of interest, for example an energy loss of electrons of the electron beam 108 diffracted from the semiconductor sample 120 or a wavelength of X-rays produced by the semiconductor sample 120 in response to the irradiation with electrons. The measurement of the property of interest can be performed by means of methods known to persons skilled in the art.
The semiconductor sample 120 is scanned by the electron beam 108. To this end, the electron beam 108 and the semiconductor sample 120 are moved relative to each other. This can be done by mechanically moving the semiconductor sample 120 or by deflecting the electron beam 108. As persons skilled in the art know, the electron beam 108 can be deflected by applying an electric field or a magnetic field exerting a force to the electrons in the electron beam 108.
In the scanning process, the electron beam 108 can be directed to a plurality of points 112 arranged along a line 111 running across the semiconductor sample 111. The line 111 may run across portions of the substrate 101 and the material layers 102, 103, 104 exposed at a cut surface of the semiconductor sample 120. Whenever the electron beam 108 impinges on one of the plurality of points 112, the detector 110 is operated to measure the property of interest. For example, for each of the points 112, an X-ray spectrum or an electron energy loss spectrum can be recorded.
The property of interest may then be analyzed in order to derive characteristics of the semiconductor sample 120. In examples of methods of inspecting a semiconductor structure according to the state of the art, information concerning a chemical composition of the semiconductor sample 120 at each of the points 112 can be derived from recorded X-ray spectra and/or recorded electron energy loss spectra. Thus, a distribution of chemical elements along the line 111 can be obtained which may then be used, for example, to investigate a homogeneity of the material layers 102, 103, 104 and/or a sharpness of interfaces between the material layers 102, 103, 104.
In addition to the motion of the semiconductor sample 120 and the electron beam 108 relative to each other performed in the scanning of the semiconductor sample 120, an additional relative motion of the semiconductor sample 120 and the electron beam 108 which is denoted as “drift” may occur, as schematically indicated by arrow 113 in FIG. 1b. The drift can be caused by mechanical displacements in the electron microscope 130 which may be generated, for example, by thermal expansion of components thereof or by a relaxation of elastic stress in components of the electron microscope 130. Another source of the drift may be alterations of static electromagnetic fields in the electron microscope 130 or an environment thereof. Such electromagnetic fields may have an influence on trajectories of electrons in the electron beam 108.
The drift may interfere with the scanning of the semiconductor sample 120. Hence, erroneous results of the scanning process can be obtained. In particular, a drift of the semiconductor sample 120 in the direction of the scan may cause erroneous measurements of thicknesses of the material layers 102, 103, 104. Whereas a drift of the semiconductor sample in the direction of the scan can yield too large measured values of layer thicknesses, a drift of the semiconductor sample in a direction opposite to that of the scan may yield too small measured values of layer thicknesses. In order to avoid such errors, it has been proposed to correct the drift by a mechanical motion of the semiconductor sample 120. To this end, speed and direction of the drift can be determined and then the sample holder 121 may be moved in the opposite direction.
A problem of the above method of compensating for the drift is that the precision of mechanical drift correction is limited. Moreover, mechanical drift correction is typically performed stepwise after measuring the property of interest at a plurality of points, for example after measurement at about thirty points. Thereby, a drift occurring during the measurement at the plurality of points is not taken into account. Hence, only a partial correction of the drift is obtained. Hence, mechanical drift correction may be insufficient for precise measurements, in particular in case of a relatively large drift rate.
In view of the above problems, there is a need for a method of inspecting a semiconductor structure allowing a more precise correction of the drift.